This invention relates to semiconductor devices and more specifically relates to a novel process for the attachment of power semiconductor die to a thermally and/or electrically conductive support.
Power semiconductor die such as diodes, MOSFETs, IGBTs and the like are normally attached to conductive lead frames or other substrates by electrically conducting materials such as epoxies, thermoplastics, solders and the like or by electrically insulative materials if electrical isolation is desired. This process is carried out sequentially for individual due, after die singulation from a wafer and is time consuming.
In accordance with the invention adhesive films which may be electrically conductive or insulative are used as the die attach material for power semiconductors. Further, such adhesive films are attached to power semiconductor wafers before the die singulation stage.
Adhesive films are now used lo bond low power integrated circuits to lead frames. In accordance with the invention, electrically conductive or insulative adhesive films are used to bond power semiconductors to substrates lead frames.
Adhesive films in the prior art are pre-cut and placed onto a substrate before the placement of die on the film. The resultant substrate/film/die assembly is then partially heat treated to promote adhesion between die/lead frame. In accordance with the invention, the adhesive film is placed onto the power semiconductor wafer before the die singulation stage. The wafer/adhesive film stack is then sawn using conventional singulation methods producing die with the adhesive film pre-attached. The sawn die/film stack is then placed onto a substrate/lead frame before re-activating the adhesive via heat treatment to promote bonding and complete the curing.
There are a number of benefits provided by the invention. Thus, conventional power semiconductor die attach involves use of epoxy or solder type adhesives in paste or liquid form. These materials often overspill from the edge of the die onto the substrate/lead frame during die bonding. This overspill limits the size of die that can be placed on the lead frame/substrate. By using an adhesive film, such overspill is eliminated. Larger die can then be placed in a package of a given size. Bond line thickness is also set by the adhesive film thickness and will be constant. Voids in the bond will also be absent.
Pre-bonding electrically conductive or (electrically isolating) adhesive film onto the power semiconductor wafer before die singulation also removes the requirement of an extra pick and place stage during assembly. Manufacturing equipment costs and cycle times are therefore reduced.